1. Field of the Invention
The present invention relates generally to a recording head for recording or printing images such as characters, figures, or graphical patterns, by applying an electric current to a ribbon or sheet (or film), or to a recording medium More particularly, the invention is concerned with a multi-layer structure including a plurality of electrodes for locally energizing the sheet or recording medium.
2. Discussion of the Prior Art
Various structures or arrangements of electrodes used for recording heads have been proposed, for applying an electric current to effect a recording operation. Examples of such recording heads are disclosed in laid-open Publication Nos. 61-35972, 58-12790, 61-230966, and 62-292461 of unexamined Japanese Patent applications. These recording heads have an array of recording electrodes formed in one plane, and a return circuit electrode layer or an array of return circuit electrodes formed in another plane parallel to the plane of the recording electrodes. The recording heads are operated such that the recording and return circuit electrodes formed in the two spaced-apart planes are held in contact with a sheet having at least an electrically resistive layer which generates heat for printing upon energization thereof. According to the arrangements as disclosed in these publications, an electrically insulating layer is disposed between the array of recording electrodes and the return circuit electrode layer or the array of return circuit electrodes. Thus, the recording head has a multi-layer structure including the insulating layer.
In the recording head having a multi-layer structure as described above, the recording and return circuit electrodes must be positioned with a high degree of accuracy, so as to provide a constant spacing between the recording electrodes and the return circuit electrode or electrodes in the direction of thickness of the recording head. This aims at preventing crosstalk between the electrodes, and assuring high degrees of uniformity and reproducibility of images printed in a matrix of dots corresponding to the recording electrodes.
Further, laid-open Publication No. 63-87264 of unexamined Japanese Patent application discloses a recording head adapted to apply an electric current to effect a recording operation, by using a heat-sensitive or thermosensive paper. On the other hand, laid-open Publication Nos. 58-104787, 61-37493, 63-160855 and 63-30279 of unexamined Japanese Patent applications disclose recording heads for applying an electric current to an ink layer or a heat-sensitive layer which is formed on the surface of a sheet, ribbon, web, roller or other support member, or which forms an inner layer of the support member. The ink layer may be an electrically conductive layer, an electrically resistive layer or electro-chemical reaction layer. The heat-sensitive layer may contain an electrolyte and produces a color due to exposure to heat. In these publications indicated above, there is no description of a multi-layered recording head in which the array of recording electrodes are spaced by a suitable distance from that of return circuit electrodes or single return electrode layer, as described above. However, the electrothermal printing by such a multi-layered recording head may be effected by using a heat-sensitive paper, electrically resistive ink layer, or heat-sensitive coloring layer, as indicated in the above publications. In this case, too, the spacing between the recording electrodes and the return circuit electrode(s) should be accurately controlled as in the case described above.
For assuring a stable permanent electrical contact between the electrodes of the recording head and the electrically resistive layer of the support member for local energization of the latter, the electrically insulating layer interposed between the recording and return circuit electrodes is required to be made of a material having a lower degree of wear resistance than that of the electrodes. The insulating layer also needs to have considerably high heat resistance, so as to prevent deterioration of its electrically insulating property and a chronological change in the thickness of the layer, due to heat generated by the electrically resistive layer, which leads to deterioration in the quality of printed or transferred images on a recording medium.
In the case of a high-speed recording operation, an ink material transferred to the recording medium is likely to blot or run, due to accumulative heat generated at the electrically resistive layer of the support member during an excess length of time, to thereby obscure the transferred images in a matrix of dots. For avoiding this, the heat generated at the electrically resistive layer should be dissipated in a short time, through the insulating layer disposed between the electrodes such that the insulating layer functions as a heatsink or heat dissipator for absorbing and dissipating the heat.
Since conventional recording heads of a multi-layer structure including recording and return circuit electrodes use an electrically insulating layer made solely of a resin material such as epoxy resin and polyimide, such recording heads are not satisfactory in terms of the heat resistance and heat dissipation.
There are also known recording heads which use an electrically insulating layer made of an ordinary glass or ceramic material. While such an insulating layer has a sufficiently high degree of heat resistance, the insulating layer is incapable of effectively dissipating heat generated during a recording operation. In addition, since the insulating layer made of the materials indicated above has a higher degree of wear resistance than that of the electrodes, the known recording head suffers from a poor electrical contact of the electrodes with the electrically resistive layer of the support member. Thus, it is difficult to obtain an optimum relative wear resistance between the insulating layer and the electrodes.
Where the electrically insulating layer is formed of mica, the layer has a high degree of heat resistance and sufficient electrically insulating capability. However, the insulating layer formed of the ordinary mica may not have sufficient uniformity in the thickness, which determines the spacing between the recording and return circuit electrodes. Consequently, the recording head having such an insulating layer suffers from deterioration in the quality of the transferred images. In addition, the insulating layer made of mica is not satisfactory in terms of heat dissipating capability.